1. Field of the Invention
The present invention is concerned wih the production of chrysanthemic aldehyde by oxidation of chrysanthemyl alcohol, which aldehyde can be converted to chrysanthemic acid.
2. Description of the Prior Art
Chrysanthemic acid, pyrethric acid and their analogues are important intermediates in the production of various synthetic pyrethrins. Various methods have been reported in the literature for preparation of chrysanthemic acid and esters thereof, none of which have been found fully satisfactory for large scale commercial adoption.
More recently an asserted simplified and less expensive method for production of chrysanthemic acid and certain precursors thereof was described in published German patent application, OLS No. 2,164,024. The therein described method involves initial production of an allene intermediate by condensing, for example, dimethylallyl alcohol with an ethynyl halide to form 2-(2'-methyl propenylidene)-3,3 dimethylcyclopropylmethanol. This intermediate allene compound is converted to chrysanthemyl alcohol by reduction with sodium in liquid ammonia. To convert the obtained chrysanthemyl alcohol to the carboxylic acid, the aforesaid publication discloses the use of chromium trioxide in dry pyridine as oxidizing agent employed in considerable excess of stoichiometric requirements. The initial reaction progresses to the aldehyde stage and only after addition of a little water and stirring over a period of several days is the chrysanthemic acid formed.
It has been observed that chrysanthemyl compounds and related precursors and analogues are sensitive to acids, bases and heat. These properties preclude the use of a majority of the known catalytic and stoichiometric chemical oxidants for desired oxidation of chrysanthemyl alcohol and evidence the need for the development of oxidative procedures applicable to these alcohols, which are economically attractive for use in practical application.
A method for oxidation of chrysanthemyl alcohol to its aldehyde with potassium dichromate has been reported in the literature (Dauben, W.G. et al, J. Org. Chem., 34, 2301 (1969) ). The described method obtains a 31% yield of the aldehyde using an aqueous acetone solution of H.sub.2 SO.sub.4 /K.sub.2 Cr.sub.2 O.sub.7. Crombie, L. et al report approximately 60% yields in the presence of a large excess of specially prepared manganese dioxide (J. Chem. Soc., 4893 (1963) ). The method employed in the aforesaid published German patent application, employing CrO.sub.3 in aqueous pyridine, and obtaining chrysanthemic acid in moderate yields, is further described in the literature; Mills, R. W. et al., J. Chem. Soc. (Perkin I), 133 (1973).
Among oxidants that have been employed or suggested for use in oxidation of simple saturated and unsaturated alcohols to corresponding aldehydes are supported and unsupported noble metal catalysts. Thus, Heyns, K. and Paulsen, H. in "Newer Methods of Preparative Organic Chemistry", 2, 303 (1963), describe catalytic oxidation of certain saturated alcohols employing unsupported platinum oxide or platinum on carbon, as well as the catalytic conversion of certain unsaturated alcohols to corresponding aldehydes over unsupported platinum oxide.
In view of the various functionalities present in chrysanthemyl alcohol sensitive to strong oxidants, such as the simultaneous presence therein of allylic hydrogen and olefinic carbon-carbon bonds, and the known rapid rearrangement of cyclopropylcarbinyl radicals and carbonium ions which are generated by certain oxidative procedures, high yields are not to be expected in the oxidation of chrysanthemyl alcohol to its aldehyde. In fact, it has been found, that chrysanthemyl alcohol decomposes even at room temperture in dilute aqueous sulfuric acid and at 125.degree. C. it is decomposed by trace amounts of chrysanthemic acid (such as would result from slight over oxidation). Vapor phase catalytic processes for oxidation of chrysanthemyl alcohol have been contraindicated because at elevated temperatures chrysanthemyl alcohol and the oxidation product aldehyde undergo thermal decomposition.
It was therefore surprising to find that exceptionally high yields of the aldehyde are selectively obtained by the oxidation of chrysanthemyl alcohol over supported platinum catalyst under the controlled conditions according to th present invention.